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CDF. Top Mass Measurements @ the Tevatron. Kirsten Tollefson Michigan State University 3 rd Top Workshop @ Grenoble. Why Measure the Mass. m t 2. ln(m H ). July 2008. 84+/-30 GeV. History. Measuring the Mass ain’t Easy. What an experimentalist sees…. What a theorist sees…. - PowerPoint PPT Presentation
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Top Mass Measurements @ the Tevatron
Kirsten TollefsonMichigan State University
3rd Top Workshop @ Grenoble
CDF
K. Tollefson, 3rd Top Workshop @ Grenoble 2
Why Measure the Mass
October 25, 2008
ln(mH) mt2
84+/-30 GeV
July 2008
K. Tollefson, 3rd Top Workshop @ Grenoble 3
History
October 25, 2008
1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 20080
50
100
150
200
250
From the EW Fits
pp colliders limit
e+e- colliders limit
CDF Run1
D0 Run1
Run1 World Average
D0 Run2 average/best result
CDF Run2 best measurement
Current world average
Year
To
p M
as
s (
Ge
V/c
2)
K. Tollefson, 3rd Top Workshop @ Grenoble 4October 25, 2008
What a theorist sees… What an experimentalist sees…
Challenges: • Combinatorics• Jet Energy Scale (JES)
Solutions: • Sophisticated analyses• In-situ Wjj calibration
Measuring the Mass ain’t Easy
K. Tollefson, 3rd Top Workshop @ Grenoble 5
Philosophy in Run II
October 25, 2008
March 2007
Mtop = 170.9 +/- 1.9 GeV/c2
using up to 2 fb-1
K. Tollefson, 3rd Top Workshop @ Grenoble 6
Non-uniform response
Diff. responseof 0p /pi+-Non-linearity
Shower, fragmentation
Jet Energy Correction
Relative using dijet balance: to make response uniform in
Absolute correction using dijet MC tuned for single particle E/P, material, and fragmentations: due to non-linear and non-compensating calorimeter
Out-of-Cone : due to energy outside cone
Multiple ppbar interactions: pileup
Underlying events due to spectators
October 25, 2008
Jet Energy Scale (JES) Review
See M. D’Onofrio’s talk from Thursday
K. Tollefson, 3rd Top Workshop @ Grenoble 7
In-situ JES Measurement
October 25, 2008
Ejet=Emeas(1+ JES *JES(Pt))
Mjj
--- Out of cone--- Absolute
K. Tollefson, 3rd Top Workshop @ Grenoble 8
Combination @ ICHEP ‘08
October 25, 2008
172.4±1.2 GeV
Run
IEa
ch c
hann
el R
un II
K. Tollefson, 3rd Top Workshop @ Grenoble 9October 25, 2008
Reconstructing the MassSee NWA talk by Joerg Meyer from Grenoble workshop in 2007
See talk by Marion Arthaud from Grenoble workshop in 2007
Wojciech Fedorko - Moriond EW, YSF 10
The template techniqueMC tt
backgroundsEvent reconstruction
DATA Event reconstructionLikelihood
Fit
03/04/2008
Generic Template Method
L+JETS
DILEPTON
SIG
SIG
BCK
BCK
K. Tollefson, 3rd Top Workshop @ Grenoble 11
• Simultaneous fit in 2 channels :– L+jets and Dilepton
• In-situ JES calibration applied in both channels
• No assumptions: – Correlations in
systematics– On likelihood shapes
October 25, 2008
DILEPTON
L+JETS
New Twist on Template Method
PRD submitted for 1.9 fb-1 result: hep-ex. 0809.4808
K. Tollefson, 3rd Top Workshop @ Grenoble 12October 25, 2008
Mtop = 172.3 +/- 1.5 (stat.+JES) +/- 1.1 (syst) GeV/c2 = 172.3 +/- 1.9 GeV/c2
Template Results with 2.7 fb-1
Measurements in traditional manner (i.e. DIL no in-situ JES)
Comparable to L+jets Matrix Element analysis with 2.7 fb-1:Mtop = 172.2 +/- 1.3 (stat.+JES) +/- 1.0 (syst) GeV/c2
= 172.2 +/- 1.7 GeV/c2
Nick van Remortel, Elba Top08 1320.05.08
Generic Matrix Element Method
Normalization depends on mt includes acceptance effects
Probability to observe a set of kinematic variables x for a given top mass
Integrate over unknown q1,q2, y
f(q) is the probability distribution that a parton will have momentum q
dnσ is the differential cross sectionContains (LO) matrix element squared
t
t
W(x,y) is the probability that a parton level set of variables y will be measured as a set of variables x
bq’
q€
Psgn (x;mt ) =1
σ (mt )dnσ (y;mt )dq1∫ dq2 f (q1) f (q2)W (x,y)
• Maximal extraction of information, but phase space integration is very CPU intensive• Additional background probability term with varying levels of sophistication
R. Demina (Rochester, D0)
K. Tollefson, 3rd Top Workshop @ Grenoble 14October 25, 2008
Event selection optimized to yield smallest expected statistical uncertainty by means of neuro-evolution:• Start with random collection of neural nets• Determine analysis sensitivity of each network (fitness function)• Discard low sensitive nets and combine topology and node weights through mutation
Neuro-evolution optimization
Converged NNet
Ref: S. Whiteson and D. Whiteson, Proceedings of the Nineteenth AnnualInnovative Appllications of Artificial Intelligence Conference, p1819-1825, July 2007K. Stanley and R. Miikulainen, Evolutionary Computation 10(2):99-127, 2002
Optimizing Dilepton Selection
KEEP
K. Tollefson, 3rd Top Workshop @ Grenoble 15
• After event select use matrix element technique
• New event selection expected statistical uncertainty improvement of 20%
October 25, 2008
Mtop = 171.2 +/- 2.7 (stat.) +/- 2.9 (syst) GeV/c2
= 171.2 +/- 4.0 GeV/c2
Dilepton Results using 2.0 fb-1
Submitted PRL: hep-ex/0807.4652
K. Tollefson, 3rd Top Workshop @ Grenoble 16October 25, 2008
L+jets Template Method using Lxy
Mtop = 180.7 +15.5/-13.4 (stat) +/- 8.6 (syst) GeV/c2
PRD 75:071102 (2007)
Lxy = average transverse decay length of B-hadron
Lxy b-jet boost Mtop
Lxy
Lxy (mm)
Lxy (cm)
K. Tollefson, 3rd Top Workshop @ Grenoble 17October 25, 2008
L+jets - Combining Lepton Pt + Lxy
Combined Result using 1.9 fb-1:Mtop = 175.3 +/- 6.2 (stat.) +/- 3.0 (syst) GeV/c2
Pt of leptonfrom W decay
Mean±1σ
Mtop = 176.7 +10/-8.9(stat) +/-3.4(syst) GeV/c2 using Lxy alone
Mtop = 173.5 +8.9/-9.1(stat) +/-4.2(syst) GeV/c2 using Lepton Pt alone
Lxy
K. Tollefson, 3rd Top Workshop @ Grenoble 18October 25, 2008
Interesting Lesson…Lxy and Lepton Pt don’t depend on JES, right?
Systematics for Lxy result using 695 pb-1
Systematics for Lxy and LepPt
results using1.9 fb-1
Event selection was affected for jets near 20 GeV threshold cut
K. Tollefson, 3rd Top Workshop @ Grenoble 19October 25, 2008
L+jets - Template Method using SLTu
Mtop = 181.3 +/- 12.4(stat.) +/- 3.5 (syst) GeV/c2
Invariant mass of lepton from W and muon from semileptonic b decay
K. Tollefson, 3rd Top Workshop @ Grenoble 20October 25, 2008
Top Mass from the Cross Section
Using NLO+NLLMtop = 167.8 +/- 5.7 GeV/c2
D0 combined Xsec
K. Tollefson, 3rd Top Workshop @ Grenoble 21October 25, 2008
Systematic Uncertainties
1. JES (for non-in situ)2. Residual JES3. b-JES4. ISR&FSR5. PDF uncertainties6. Generator &
modeling7. Multiple interactions
(a.k.a Pile-up)8. Background
fraction & Shape9. Lepton Energy
scale
Current list: Working on:1. MC generators: checking
against NLO MCs2. Color reconnection –
more later
Systematics, Systematics, Systematics
Systematics for Template Analysis using 2.7 fb-1
K. Tollefson, 3rd Top Workshop @ Grenoble 22October 25, 2008
• Use jets from hadronic W resonance in messy ttbar environment to measure the average response of jets• In-situ measured JES does not fully measure shifts in JES scale along different parameter space curves (e.g. jet Pt and η)
• Even for in-situ measurement still evaluate JES uncertainty using standard procedure by shifting JES +/- 1σ• Must re-compute acceptances and shapes for both ttbar
and backgrounds
Residual JES
K. Tollefson, 3rd Top Workshop @ Grenoble 23October 25, 2008
• Derive JES from W daughter jets, but b jets carry most Mtop information
• Study 3 components due to difference between b and q jets:• Semi-leptonic branching ratios
• Move b anc c BRs together by +/- 1σ• B fragmentation uncertainties
• Reweight to LEP/SLD Bowler parameters• Calorimeter response uncertainties
• Shift b-jet energies by +/-1% then re-run PEs
JES for b quarks
K. Tollefson, 3rd Top Workshop @ Grenoble 24
Remaining issues
• Pythia 6.4 includes:– PT ordered showering which
allows for parton showers to interact with the underlying event
– new color reconnection models
• Study by Wicke and Skands on toy top mass measurement see ~1 GeV differences – see Wicke and Skands,
arXiv 0807.3248 and hep-ph/0703081
October 25, 2008
Color Reconnection Studies
K. Tollefson, 3rd Top Workshop @ Grenoble 25
Remaining issues
Virtuality ordered PS (old) PT ordered PS (new)• Results:
– Total spread +/- 1 GeV• CDF and D0 are both
working on studying these new Pythia tunes within our analysis methods
October 25, 2008
Color Reconnection Studies
K. Tollefson, 3rd Top Workshop @ Grenoble 26October 25, 2008
Many analysis techniques to measure top massTevatron still learning new tricksTest bed for methods for LHC
Mass is now known to 0.7% Need to be absolutely confident in the systematic uncertainties we quote – working hard on this Tevatron top mass results at: CDF -
http://www-cdf.fnal.gov/physics/new/top/public_mass.html
D0 - http://www-d0.fnal.gov/Run2Physics/top/top_public_web_pages/top_public.htm
and one final question…
Final Thoughts…
What mass are we measuring anyway?
K. Tollefson, 3rd Top Workshop @ Grenoble 28October 25, 2008
ISR & FSR
• Use dedicated Pythia samples with increased/decreased amount of ISR/FSR• Variations in pythia parameters are determined by studying dimuon events only sensitive to ISR• FSR parameters are varied within similar bounds, assuming the physics is similar• Extrapolation from DY data to ttbar events is large• Pythia parameters control mainly the soft part of FSR, might overlook hard (NLO type) radiation
Will try to pin down this uncertainty bandby using new data and adding higher mass points
Currently changed to samples where ISR and FSR are simultaneously increased or decreased
ISR/FSR
Un-ki Yang, Manchester 29
b-JES using Z(bb)• Di b-jets with Et>22 GeV, ΔΦ>3.0,Et
(3rd)<15 GeV using SVT impact parameter trigger at L2 • To measure data/MC b-JES
b-JES=0.974 ±0.011(stat) +-0.014+0.017 (syst)
Has not applied to b-JES in top mass • different cone size• different pt spectrum
K. Tollefson, 3rd Top Workshop @ Grenoble 30October 25, 2008
Problem
• Our MC simulates only one parton-parton interaction per event• We add additional min bias events according to our lumi profile and determine JES correction• In ttbar events our MC still underestimates the amount of multiple parton- parton interactions in each collision• How does this propagate into an Mtop uncertainty ?
B-Jet Et increases with ~200 MeVFor each additional vertex
• We find mean of ~2 vertices per event in our current 2 fb-1 dataset• We know that B-Jets affect Mtop most• We know how a 1% bjet ET increase affects Mtop
• Total effect is O(200 MeV) on Mtop
Multiple Interactions (Pile-up)
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